An image sensor usually includes an array of pixels, and can be fabricated using complementary metal-oxide-semiconductor (CMOS) processes. A CMOS image sensor may be illuminated from a front side (or a top side) of a silicon die. Because various features related to the CMOS processes, such as metalization, polysilicon, and diffusions, are typically made on the front side of the silicon die, the pixel areas of a front-side illuminated image sensor are often partially obscured, which results in a loss of light reaching photosensitive regions within the pixels and a reduction of the overall sensitivity of the image sensor. A backside illuminated (BSI) CMOS image sensor allows light to be collected from a back side (or a bottom side) of the sensor. The backside of the sensor is relatively unobstructed by many dielectric and/or metal layers involved in the CMOS processes, and thus the overall sensitivity of the image sensor may be improved.